This invention relates to a current protector, particularly an organic resin-made chip type current protector and processes for producing the same.
Current protectors are used for protecting electronic devices from over-current acting as an automatic circuit-interrupting device. The current protector used in the present invention is connected in series to an electric circuit and is subjected to blowing of a fusible link in the current protector under over-current conditions so as to protect devices by breaking an electric current thereafter. Such an element is generally called a fuse. When the term "fuse" is used, the element should satisfy the required properties specified in various regulations. But with recent diversification of electronic devices, there appear current protectors having properties different from those specified in the regulations for the fuse. In the present invention, the term "current protector" is used in a wide meaning including conventional fuses and having the properties and operational mechanism mentioned above.
As over-current protecting devices, there can be used as electronic switches using thyristors or transistors in addition to the above-mentioned current protectors. But such devices were not always suitable for devices which require miniaturization and a low consuming electric power such as portable devices operated by a battery due to an increase in circuit parts and an increase in electric power consumed by a protective circuit.
As current protectors having improved properties, JP-A-60-143544 disclosed a current protector (or fuse) comprising a ceramic substrate and formed thereon a three-layered electrical conductor comprising a first layer of silver or silver-palladium, a second layer of nickel and a third layer of solder or tin so as to improve clearing (or blowing) characteristics at the time of soldering. It was also disclosed therein to cover the electrical conductor surface with an incombustible (or fire retardant) resin such as a silicon resin. But such a structure wherein the fuse was formed on the ceramic substrate which is small in thermal resistance, was high in heat dissipation and had a problem in that a current value for blowing often changed depending on ambient temperatures, even if covered with the incombustible (or fire retardant) resin as disclosed by JP-A-60-143544.
In order to solve the problem of using the ceramic substrate, it was proposed to use organic resin-made insulating substrates. But when an epoxy resin, a phenol resin, a polyimide resin, etc. were used as the substrate, there were problems in fuming and combustion.
JP-A-5-166454 disclosed the use of a fluorine resin as the insulating substrate so as to lower thermal conductivity compared with ceramic and to improve blowing accuracy of the fuse. The surface of fuse was also covered with a silicone resin (rubber).
On the other hand, the fusible link of the fuse was formed by printing or plating (JP-A-63-141233). But the accuracy for forming the fusible link was low and particularly it was difficult to control the thickness of the fusible link. Thus, it was difficult to make scattering of resistance value of the fusible link between lots within 30%.
According to JP-A-5-166454, the fusible link was formed by forming a thin metal layer by plating, followed by etching. Such a fusible link was excellent in clearing (or blowing) characteristics, but poor in controlling the uniform thickness of plated layer due to difficulty in controlling of plating conditions such as the composition of a plating bath, etc. Thus, it was difficult to make the scattering of resistance value between lots within 30%. Further, since the fusible link was formed by etching of plated thin metal layer according to JP-A-5-166454, it was impossible to obtain sufficient connection reliability for a long period of time when subjected to an accelerated test of heat cycle test. In addition, according to JP-A-5-166454, since the surface of fusible link was covered with the silicone rubber, the silicone rubber was often damaged and caused slight fuming for 1 or 2 seconds due to high temperatures at the time of blowing depending on over-current conditions.